Electrical machines of the type mentioned above are described, for instance, in international patent application WO 97/45919. The cables forming the stator winding are suitably high-voltage cables of substantially the same type as those used for power distribution, i.e. PEX cables (PEX=cross-linked polyethylene). However, contrary to power distribution cable, the winding has no metal sheath which normally surrounds such a power distribution cable. The cable thus comprises substantially only the electrical conductor and at least one semiconducting layer on each side of the insulating layer. These machines can be built for extremely high voltages, 800 kV and higher, and for extremely high powers, up to 1500 MW.
When manufacturing the winding for such a stator, the stator is usually provided with mainly closed slots, the cable being drawn axially through the slots. In electrical machines intended for these extremely high voltages, several hundred kilovolt, the total length of cable in the stator winding will be considerable, up to several kilometers per phase. In such electrical machines the slots for the stator winding in the stator core must also be made very deep to ensure sufficient space for the required number of winding turns in the slot, and this in turn leads to the teeth that separate the slots becoming very long. Furthermore, it is essential for the function of these machines that the outermost semiconducting layer of the cable is not damaged during winding of the machine, since this layer is vital for enclosing the electric field in the winding.
In the case of machines intended for lower power, up to 30–40 MW, and lower voltages, to which the present invention relates, the stator winding is composed of a cable with considerably smaller diameter than the winding in a machine for high voltage and high power. In a generator for wind power plants of the type described, for instance, in international patent applications SE99/00943 and SE99/00944 and in published international application WO99/28919, the generator is driven directly and operates at very low speed, 10–25 rpm, which in turn means that it must be designed with a large number of poles. Many poles result in large diameter and thus high weight.